EVOH is excellent in transparency, gas barrier property (oxygen barrier property and the like), aroma retaining property, solvent resistance, oil resistance, mechanical strength and the like, and are typically formed into films, sheets, bottles and the like, which are widely used for packaging materials such as food packaging materials, medical drug packaging materials, industrial chemical packaging materials, agricultural chemical packaging materials.
Such an EVOH is typically prepared by preparing an ethylene-vinyl ester copolymer through copolymerization of ethylene and a fatty acid vinyl ester such as vinyl acetate, and saponifying the ethylene-vinyl ester copolymer in an alcohol solvent such as methanol in the presence of an alkali catalyst under higher-temperature higher-pressure conditions.
It is a conventional practice to convert the EVOH alcohol solution prepared under the higher-temperature higher-pressure conditions through the saponification into an EVOH water/alcohol mixture solution which is stable at an ordinary pressure, extrude the EVOH water/alcohol mixture solution in a coagulation bath mainly containing water at a lower temperature for solidification in the form of strands, cut the strands for pelletization, and dry the resulting pellets to provide a product.
In this EVOH preparation process, however, the alcohol is liable to run into the coagulation bath when the EVOH water/alcohol solution is extruded into the strands in the coagulation bath. The alcohol is evaporated in air, thereby impairing a working environment.
To solve this problem, there have been proposed: an EVOH resin production process (see, for example, Patent Document 1) which includes the steps of feeding an EVOH alcohol solution into an apparatus and bringing the EVOH alcohol solution into contact with water in the apparatus to replace the alcohol with water to provide an EVOH composition having a lower alcohol content (Step 1), and melt-kneading the EVOH composition by means of an extruder to provide an EVOH resin (Step 2); and an EVOH pellet production process (see, for example, Patent Document 2) which includes the steps of feeding an EVOH alcohol solution into an apparatus and bringing the EVOH alcohol solution into contact with water in the apparatus to replace the alcohol with water to provide an EVOH composition (Step 1), pelletizing the EVOH composition (Step 2), drying the resulting pellets (Step 3), melt-kneading the pellets having a reduced water content by means of an extruder (Step 4), and cutting an EVOH extruded from the extruder into pellets (Step 5).
These production processes make it possible to recover the alcohol removed from the EVOH alcohol solution without emanation of the alcohol, thereby eliminating the possibility of impairing the working environment.    Patent Document 1: JP-A-2002-284811 (US2002/100997A1)    Patent Document 2: W02004/009313 (US2006/108703A1)
As specified in Patent Document 1, the EVOH composition provided in Step 1 of the production process disclosed in Patent Document 1 has a water content of 10 to 1000 parts by weight based on 100 parts by weight of the EVOH and, if the water content is less than 10 parts by weight, the EVOH composition has unsatisfactory fluidity in the container. A preferred example of the apparatus for use in Step 1 is a tower type apparatus which ensures efficient replacement of the alcohol with water, continuous processing and excellent productivity. Where the tower type apparatus is used in Step 1, however, the unsatisfactory fluidity makes it difficult to fluidize the EVOH composition in the apparatus and feed out the EVOH from the apparatus. Therefore, if the alcohol content is reduced to 0 to 10 parts by weight based on 100 parts by weight of the EVOH, it is difficult to reduce the water content to 100 parts by weight or less. In examples shown in Patent Document 1, the water content of the EVOH composition is 105 parts by weight based on 100 parts by weight of the EVOH, because the tower type apparatus is used. Therefore, the EVOH composition thus having a higher water content should be subsequently melt-kneaded in Step 2. This results in separation of water during the feeding of the EVOH composition into the extruder and unstable kneading. Since a greater amount of water is evaporated, the extruder requires a plurality of vents, or requires a higher L/D ratio or a lower extrusion rate to increase a retention time during which the EVOH composition is retained in the extruder in order to sufficiently remove the water. This disadvantageously increases the costs of the apparatus, and fails to increase the productivity.
The process according to the invention disclosed in Patent Document 2, which aims to alleviate this problem, includes the step of pelletizing the higher water content EVOH composition prepared in Step 1 and drying the resulting pellets between Step 1 and Step 2 of the process disclosed in Patent Document 1, so that an EVOH composition having a reduced water content is melt-kneaded in the subsequent step. However, the process disclosed in Patent Document 2 requires a device for pelletizing the EVOH composition prepared in Step 1, and a device for drying the resulting pellets, thereby leading to disadvantageous production costs.
Further, the drying step requires a great amount of heat for the removal of the great amount of water contained in the EVOH, and is liable to thermally degrade the EVOH pellets retained in a dryer for a long period of time. This increases the possibility of contaminating a product with foreign matter of the thermally degraded EVOH.